More than a century after the end of the Gold Rush, scientists have found new evidence of its toxic legacy: high levels of poisonous mercury in bass and catfish in two northern Sierra Nevada watersheds.

The finding by the U.S. Geological Survey, contained in a report released yesterday, prompted California officials to warn people about eating fish from the Bear and Yuba Rivers.

The study is based on 141 samples of fish, including catfish, bass, trout, and sunfish, collected last year from reservoirs and streams in Nevada, Placer and Yuba counties. The reservoirs included Englebright Lake, Scotts Flat Reservoir, Rollins Lake, Lake Combie and Camp Far West Reservoir.

Numerous studies have assessed the environmental effects of mining operations in the Sierra, but most have focused on acidic water and heavy metals.

"Our fish survey is part of the first comprehensive investigation in the Sierra Nevada region of mercury distribution in water, sediment, and biota, and the potential risks to human health and ecosystems," said Charlie Alpers, a USGS chemist and the study's chief scientist.

The survey found high concentrations of liquid elemental mercury, or quicksilver, in the Bear River and Yuba River watersheds, where it was used extensively from the early gold mining days until the early 1900s.

Hydraulic mining, which was invented in the 1850s in the Bear River watershed, is believed to be the culprit. Mercury was used to separate gold from ore and to create an amalgam that could later be separated.

"The legacy of the Gold Rush is finally being more appreciated," Alpers said. "It's another little gift from our ancestors that we haven't quite unwrapped yet. I'm sure the gold miners who poured mercury by the flask into sluice boxes didn't want to hurt anyone. They just wanted to find more gold."

He estimates that between 3 million to 8 million pounds of mercury was dumped into the Sierra environment through hydraulic mining.

The levels are high enough to merit concern, scientists said, but for the most part were below the level where the federal government must take action.

Mercury levels in bass ranged from 0.20 to 1.5 parts per million (ppm). Channel catfish had mercury levels from 0.16 to 0.75 ppm. Brown trout had levels from 0.02 to 0.43 ppm. Sunfish had levels ranging from less than 0.10 to 0.41 ppm.

"Elemental mercury, the kind you can see, is only one part of the problem," said Jason May, a USGS biologist. "It is the presence of methylmercury, the organic form of mercury that accumulates in organisms, that will be of most concern."

Methylmercury is known to increase in concentration as it moves up the food chain. Concentrations tend to be highest in predatory fish like bass and brown trout.

Federal scientists have submitted their data to the California Environmental Protection Agency and county health officials, who will determine whether to formally notify the public about the potential risks from consuming fish in these areas.

Twelve bodies of water in California, including San Francisco Bay, Clear Lake and Lake Berryessa, have health advisories related to high levels of mercury in fish. But there are currently no such advisories in California's historic gold mining areas.

Children and fetuses are the most susceptible to mercury poisoning because their nervous systems are still growing. Eating fish contaminated with mercury can affect a child's behavior, causing deficiencies in motor skills, language, and attention span.

Hirsch, of Cal EPA, said that the mercury levels found in the Sierra are comparable to the levels previously found in San Francisco Bay, the Sacramento Delta, reservoirs in Santa Clara County and lakes in the Coast Mountain Ranges, where mercury was mined before being brought to the Sierra for gold mining.